1. Field of the Invention
The present invention relates to a laser oscillator and a laser amplifier, and particularly to a solid-state laser incorporating a gain medium including a rare earth dopant species, and more particularly to a fiber laser.
2. Description of the Related Art
Rare earth doped lasers are key devices in optical data transmission systems, in laser processing systems and in laser medical systems. Examples of rare earth dopant species are erbium (Er), holmium (Ho), thulium (Tm), and praseodymium (Pr). The rare earth doped lasers include glass lasers (for example, fiber lasers), solid laser oscillators, and laser amplifiers (for example, fiber amplifiers).
Erbium ions are excited by light from an exciting light source having a wavelength of 1.48 xcexcm (microns) or light from an exciting light source having a wavelength of 0.98 xcexcm (microns). When excited, they are raised to an upper laser energy level of 4I13/2, from which stimulated transition to a lower energy level of 4I15/2 is to be produced. Production of this stimulated transition causes an erbium doped optical fiber to exhibit gain spectra range from 1.53 xcexcm to 1.57 xcexcm with 1.55 xcexcm as a central wavelength. Transition of erbium ions from 4I13/2 to 4I15/2 has fluorescent spectra range from 1.50 xcexcm to 1.60 xcexcm. This means that a satisfactory gain can be obtained at 1.60 xcexcm band by optimally setting wavelength of exciting light and fiber parameters, such as, the strength and length of fiber and the density of erbium dopant.
Recently, attempts have been reported to make a shift of the peak of gain spectra toward a longer wavelength side with the fluorescent band (in the neighborhood of 1.60 xcexcm). As an example of an attempt to achieve such a shift, Ono et al., xe2x80x9cGain-Flattened Er3+-Doped Fiber Amplifier for a WIDM Signal in the 157-1.60xcexcm Wavelength Region,xe2x80x9d IEEE Photonics Technology Letters, Vol. 9, No. 5 May 1997 pages 596-598 describe that 200 m is the optimum length of silica-based Er3+-doped fiber for constructing an Er3+-doped fiber amplifier (EDFA), which exhibits uniform amplification characteristic in the 1.57-1.60 xcexcm wavelength region.
With regard to erbium, the above-mentioned shift of gain spectra, which is hereinafter referred to as xe2x80x9ca gain shiftxe2x80x9d, is hey technique in accomplishing amplification over a wide wavelength band. The gain shift technique is very important in next generation wavelength division multiplexing (WDM) data transmission systems.
With regard to any other stimulated transition of erbium than the transition from 4I13/2 to 4I15/2, there is no example of an attempt to make a gain shift. This is because the above-mentioned gain shift technique is applicable to the stimulated transition to the ground state level, only. Gain shift technique has not been developed with regard to stimulated transitions to energy levels other than the ground state level, which are regarded as important stimulated transitions for industrial applications. Examples of such important stimulated transitions are a stimulated transition of thulium (Tm) from 3F4 to 3H4 (1.47 xcexcm band), and a stimulated transition of praseodymium (Pr) from 1G4 to 3H5 (1.3 xcexcm band), which are suitable in optical data transmission systems because those stimulated transitions have wavelengths that belong to a wavelength band with a low transmission loss through an optical fiber. Other examples are a stimulated transition of erbium (Er) from 4I11/2 to 4I13/2 (2.7 xcexcm band), and a stimulated transition of holmium (Ho) from 5I6 to 5I7 (2.9 xcexcm band), which may be used in medical field as a laser surgical knife because of extremely high capability of absorbing the OH. group in organic compounds constituting a human body. A gain spectrum or oscillatory spectrum inherent with the transitions are substantially the same as a fluorescent spectrum (a spectrum of emission cross section) of emission due to transition from an upper level to a lower level, and it is peculiar to the material.
FIG. 9 illustrates the energy level structure of thulium (Tm) dopant within fluoride-based glass and a conventional exciting technique or level scheme. Fluoride-based glass fiber with a thulium doped core exhibits a fluorescent spectrum, having a range of wavelengths from 1.45 xcexcm to 1.50 xcexcm with 1.47 xcexcm as a central of wavelength, due to a transition from 3F4 to 3H4. The gain medium is pumped by light derived from an exciting light source having a 1.05 xcexcm wavelength band (ranging from 1.04 to 1.07 xcexcm) and raised from a ground level to a 3F4 level with a high efficiency through two-stage excitation scheme (involving excited state absorption) as illustrated in FIG. 9, resulting in production of laser amplification and laser oscillation at 1.47 xcexcm band. This technique is disclosed in JP-B2 2688303.
Wavelength dependency of the gain fiber amplifier gain is substantially identical with the fluorescent spectrum and has a single ridge profile having its peak at 1.47 xcexcm.
Accordingly, a need remains to provide a laser amplifier and a laser oscillator, which are capable of carrying out amplification over a longer wavelength band than ever in each of the before listed important transitions.
An object of the present invention is to provide a laser amplifier and a laser oscillator incorporating a transition to an energy level higher than a ground level, which amplifier and oscillator are capable of carrying out amplification over a longer wavelength band than ever by utilizing a gain shift technique.
According to the present invention, there is provided a laser amplifier and/or oscillator comprising:
a gain medium including rare earth dopant in a host medium, ions of said rare earth dopant having an energy level structure including a ground level and a pair of laser upper level and laser lower levels between which a stimulated transition is to be produced after production of a population inversion between said pair of laser upper and laser lower levels, said pair of laser upper and laser lower levels being higher than said ground level;
a first exciting light source coupled to said gain medium for introducing first exciting light to said gain medium to produce said population inversion; and
a second exciting light source coupled to said gain medium for introducing second exciting light source to said gain medium to raise ions of said rare earth dopant from said ground level to said laser lower level, said second exciting light having a wavelength band different from a wavelength band of said first exciting light.